Devices used to examine the surface of a semiconductor wafer commonly use a device known as an edge handling chuck to retain the wafer during inspection. Modern examination techniques often entail incrementally inspecting the surface of the wafer with a probe that moves radially while the wafer rotates, much like a phonograph needle passes over a phonograph record. As may be appreciated, the greater the speed of rotation, the larger the number of sample wafers which may be examined for a given time.
An edge handling chuck is one which simply secures the wafer at three or four or more fixed equidistant points at the wafer's periphery and holds the wafer at these fixed points during rotation. U.S. Pat. No. 6,217,034, which is incorporated herein by reference, describes an example of an edge handling chuck having a plate more or less cylindrical in shape that holds a wafer using multiple spring load edge wafer clamps. Air passes through a center hole in the cylindrical plate and is dispersed to the atmosphere using multiple pressure relief openings in the cylindrical plate. The air pressure in the gap between the chuck surface and the back side of the wafer supports the wafer against its tendency to sag due to its own weight and the effects of rapid rotation. This edge handling chuck design provides acceptable chuck performance at rotation speeds up to about 1500 rpm. An edge handling chuck is often desirable since it typically produces less particle contamination than chucks that secure the wafer by forces exerted on the wafer backside. Edge handling chucks can also provide better control of wafer flatness for a more uniform depth of focus across the wafer surface.
For current wafer inspection systems it is desirable to maintain a stable surface for rotation speeds up to and greater than approximately 2250 rpm. The airflow supply rate typically increases 4 to 5 times to provide enough wafer support at this speed of rotation. The increased amount of air and higher air velocities may deform the wafer and cause the loss of thousands of particles, which contaminate the back surface of the wafer way above an acceptable level. Future applications may require even higher rotation rates, perhaps as great as 3000 rpm or even greater.
It is within this context that embodiments of the present invention arise.